The present application claims priority to Japanese Application(s) No(s). P2000-308300 filed Oct. 6, 2000, and P2000-308313 filed Oct. 6, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a cathode active material capable of reversibly doping/dedoping lithium and to a non-aqueous electrolyte cell employing this cathode active material.
2. Description of Related Art
Recently, with drastic progress in the art of electronic equipment, investigations into a rechargeable secondary cell, as a power source that may be used conveniently and economically for prolonged time, are proceeding briskly. Among typical secondary cells, there are a lead storage cell, an alkali storage cell and a non-aqueous electrolyte secondary cell.
Among the aforementioned secondary cells, a lithium ion secondary cell, as a non-aqueous electrolyte secondary cell, has advantages such as high output or high energy density.
A lithium ion secondary cell is made up of a cathode and an anode, each having an active material capable of reversibly doping/dedoping at least lithium ions, and a non-aqueous electrolyte. The charging reaction of the lithium ion secondary cell proceeds as lithium ions are deintercalated into an electrolyte solution at the cathode and are intercalated into the anode active material. In discharging, reaction opposite to that of the charging reaction proceeds, such that lithium ions are intercalated at the cathode. That is, charging/discharging is repeated as the reaction of entrance/exit of lithium ions from the cathode into and from the anode active material occurs repeatedly.
As the cathode active material of the lithium ion secondary cell, LiCoO2, LiNiO2 or LiMn2O4 is used because these materials have a high energy density and a high voltage. However, these cathode active materials, containing metal elements of low Clark number in their composition, suffer from high cost and supply instability. Moreover, these cathode active materials are higher in toxicity and affect the environment significantly. So, there is presented a demand for a novel substitution material usable as a cathode active material.
Proposals have been made for use of LiFePO4 having an olivinic structure, as a cathode active material for a lithium ion secondary cell. LiFePO4 has a volumetric density as high as 3.6 g/m3 and generates a high potential of 3.4V, with its theoretical capacity also being as high as 170 mAh/g. Additionally, LiFePO4 contains an electrochemically dedopable Li at a rate of one atom per Fe atom, in its initial state, and therefore is promising as a cathode active material for a lithium ion secondary cell. Moreover, LiFePO4 includes iron, as an inexpensive material plentiful in supply, in its composition, and therefore is less costly than any of the aforementioned materials, that is LiCoO2, LiNiO2 or LiMn2O4.
However, since LiFePO4 has only low electronic conductivity, the internal resistance of the cell may occasionally be increased if LiFePO4 is used as a cathode active material. If the internal resistance of the cell is increased, the polarization potential on cell circuit closure is increased to decrease the cell capacity. Additionally, since the true density of LiFePO4 is lower than that of the conventional cathode material, the active material charging ratio cannot be increased if LiFePO4 is used as the cathode active material, such that the call cannot be increased sufficiently in energy density.
So, a proposal has been made of employing, as a cathode active material, a composite material of a compound represented by the general formula LixFePO4, where 0 less than xxe2x89xa61, having an olivinic structure, and a carbon material for its superiority in electronic conductivity. This composite material is referred to below as an LiFePO4 composite material.
Meanwhile, if an impurity is left over in the LixFePO4 carbon composite material, as a cathode active material, the cell characteristics are lowered, because the impurity fails to contribute to the cell reaction. For improving the cell characteristics, it is necessary to prepare the LixFePO4 carbon composite material not containing residual impurity, that is to synthesize the LixFePO4 carbon composite material in a single phase.
For preparing the LixFePO4 carbon composite material, such a method has been proposed which consists in mixing starting materials for synthesis of LixFePO4, milling the resulting mixture, sintering the milled product and adding a carbon material at an optional time point to the starting materials for synthesis.
It is however difficult to realize a smooth reaction for synthesis in the sintering process, such that there lacks at present a technique of synthesizing the LixFePO4 carbon composite material in a single phase and therefore a non-aqueous electrolyte cell employing the LixFePO4 carbon composite material synthesized in a single phase has not been realized.
It is therefore an object of the present invention to provide a method for the preparation of a cathode active material having a superior cell capacity through reliable single-phase synthesis of a compound represented by the general formula LixFe1-yMyPO4 and a carbon material and a method for the preparation of a non-aqueous electrolyte cell having a high cell capacity.
In one aspect, the present invention provided a method for preparing a cathode active material including a mixing step of mixing starting materials for synthesis of a compound represented by a general formula LixFe1-yMyPO4, where M is at least one selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B and Nb, with 0.05xe2x89xa6xxe2x89xa61.2 and 0xe2x89xa6yxe2x89xa60.8, a milling step of milling a mixture obtained in the mixing step, a compressing step of compressing the milled mixture obtained in the milling step to a preset density and a sintering step of sintering the mixture compressed in the compressing step. A carbon material is added in any of the above steps previous to the sintering step and the density of the mixture is set in the compressing step to not less than 1.71 g/cm3 and not larger than 2.45 g/cm3.
In the method for preparing the cathode active material, described above, there is provided the compressing step between the milling and sintering steps of compressing the milled mixture, that is the milled starting materials for synthesis of the cathode active material, to a preset density, that is to not less than 1.71 g/cm3 and not larger than 2.45 g/cm3. This diminishes the gap between the particles of the mixture, that is the starting materials for synthesis of the cathode active material, charged into the sintering step, thereby assuring a sufficient area of contact of the particles of the starting materials for synthesis. By carrying out the sintering step as a sufficient contact area is maintained between the starting materials for synthesis, the synthesis reaction is improved in reaction efficiency to realize single-phase synthesis of the cathode active material, that is the composite material composed of LixFe1-yMyPO4 and carbon. So, with the manufacturing method for the cathode active material, it is possible to produce a cathode active material which may assure a high cell capacity.
That is, with the method for preparing the cathode active material, according to the present invention, there is provided, between the milling step and the sintering step, a step of compressing the milled mixture, that is milled starting materials for synthesis of the cathode active material, to a preset density, that is to not less than 1.71 g/cm3 and not larger than 2.45 g/cm3, thus realizing single-phase synthesis of the cathode active material, that is LiFePO4 carbon composite material.
So, with the present manufacturing method for the cathode active material, there may be provided a manufacturing method for the cathode active material for a cell having a high cell capacity through single-phase synthesis of the cathode active material.
In another aspect, the present invention provides a method for the preparing a non-aqueous electrolyte cell having a cathode including a cathode active material, an anode including an anode active material and a non-aqueous electrolyte, wherein the cathode active material is produced by a mixing step of mixing starting materials for synthesis of a compound represented by the general formula LixFe1-yMyPO4, where M is at least one selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B and Nb, with 0.05xe2x89xa6xxe2x89xa61.2 and 0xe2x89xa6yxe2x89xa60.8, a milling step of milling a mixture obtained in the mixing step, a compressing step of compressing the milled mixture to a preset density and a sintering step of sintering the mixture compressed in the compressing step, a carbon material being added in any of the above steps previous to the sintering step. The density of the mixture is set in the compressing step to not less than 1.71 g/cm3 and not larger than 2.45 g/cm3.
In the method for producing the non-aqueous electrolyte cell, described above, there is provided, in producing the cathode active material, the compressing step between the milling and sintering steps of compressing the milled mixture, that is the milled starting materials for synthesis of the cathode active material, to a preset density, that is to not less than 1.71 g/cm3 and not larger than 2.45 g/cm3. This diminishes the gap between the particles of the mixture, that is the starting materials for synthesis of the cathode active material, charged into the sintering step, thereby assuring a sufficient area of contact of the particles of the starting materials for synthesis. By carrying out the sintering step as a sufficient contact area is maintained between the starting materials for synthesis, the synthesis reaction is improved in reaction efficiency to realize single-phase synthesis of the cathode active material, that is the composite material composed of LixFe1-yMyPO4 and carbon. So, with the manufacturing method for the non-aqueous electrolyte cell, it is possible to produce a non-aqueous electrolyte cell having a high cell capacity.
That is, with the manufacturing method for the non-aqueous electrolyte cell, according to the present invention, there is provided, between the milling step and the sintering step, a step of compressing the milled mixture, that is milled starting materials for synthesis of the cathode active material, to a preset density, that is to not less than 1.71 g/cm3 and not larger than 2.45 g/cm3, thus realizing single-phase synthesis of the cathode active material, that is LiFePO4 carbon composite material.
So, with the present manufacturing method for the non-aqueous electrolyte cell, there may be provided a manufacturing method for the cathode active material for a cell having a high cell capacity through single-phase synthesis of the cathode active material.
In still another aspect, the present invention provides a method for preparing a cathode active material including a mixing step of mixing starting materials for synthesis of a compound represented by the general formula LixFe1-yMyPO4, where 0.05xe2x89xa6xxe2x89xa61.2, 0xe2x89xa6yxe2x89xa60.8, and M is at least one selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B and Nb, a milling step of milling a mixture obtained in the mixing step, and a sintering step of sintering the mixture milled in the milling step. A carbon material is added in any of the above steps and, following the milling step, the tap density of the starting materials for synthesis is set to not less than 0.4 g/cc and not larger than 2.0 g/cc.
In the method for preparing the cathode active material according to the present invention, in which the tap density of the starting materials for synthesis following the milling step is prescribed as described above, the starting materials for synthesis are comminuted sufficiently thus assuring a sufficient contact area of the particles of the starting materials for synthesis during the sintering step. Thus, with the present manufacturing method for the cathode active material, the synthesis reaction in the sintering step is improved in reaction efficiency, thereby yielding a composite material of LixFe1-yMyPO4 and the carbon material, that is a cathode active material not containing impurities. It should be noted that the milling means comminution and mixing performed concurrently.
In yet another aspect, the present invention provides a method for preparing a non-aqueous electrolyte cell having a cathode including a cathode active material, an anode including an anode active material and a non-aqueous electrolyte, wherein, for producing the cathode active material, a mixing step of mixing staring materials for synthesis of a compound represented by the general formula LixFe1-yMyPO4, where 0.05xe2x89xa6xxe2x89xa61.2, 0xe2x89xa6yxe2x89xa60.8, and M is at least one selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B and Nb, a milling step of milling a mixture obtained in the mixing step, and a sintering step of sintering the mixture milled in the milling step, are carried out. A carbon material is added in any of the above steps and, following the milling step, the tap density of the starting materials for synthesis is set to not less than 0.4 g/cc and not larger than 2.0 g/cc.
In the manufacturing method for the non-aqueous electrolyte cell according to the present invention, a composite material of LixFe1-yMyPO4 and the carbon material can be synthesized reliably in a single step, thus yielding a non-aqueous electrolyte cell having superior cell characteristics, such as cell capacity or cyclic characteristics.
The method for preparing the cathode active material of the present invention comprises a mixing step of mixing starting materials for synthesis of a compound represented by the general formula LixFe1-yMyPO4, where 0.05xe2x89xa6xxe2x89xa61.2, 0xe2x89xa6yxe2x89xa60.8, and M is at least one selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B and Nb, a milling step of milling a mixture obtained in the mixing step, and a sintering step of sintering the mixture milled in the milling step. Moreover, a carbon material is added in any of the above steps while the tap density of the starting materials for synthesis is set to not less than 0.4 g/cc and not larger than 2.0 g/cc after the milling step. Thus, the starting materials can be commuted sufficiently, so that the sufficient contact area in which starting material contacts with each other can be assured. That is, according to the method for the cathode active material, a smooth reaction for synthesis in the sintering step can be achieved, and the LixFe1-yMyPO4 carbon composite material can be synthesized in a single phase. This enable the cathode active material having superior cell characteristics, and free of impurities.
Also, according to the method for preparing non-aqueous electrolyte cell, the non-aqueous electrolyte cell is produced with the use of so-produced cathode active material. This enable the non-aqueous electrolyte cell which is superior in its cell characteristics, such as cell capacity or cyclic characteristics.